1. Field of the Invention
One aspect of the present invention relates to short fibers of anisotropic polymers, and to processes for forming such fibers. Another aspect of this invention relates to composites comprising a thermoplastic and/or thermoset polymer and short fibers of anisotropic polymers.
2. Prior Art
The use of short fibers as reinforcing components in various types of composites is known. For example, such composites are described in L.J. Broutman and R.H. Krock, "Modern Composite Materials", Addison-Wesley, Boston, MA (1967). Such fibers may be composed of inorganic materials or may be composed of organic materials such as polymers. Representative of inorganic materials which may be used in the fabrication of fibers for use in known composites are asbestos, glass, boron, graphitic carbon, amorphous carbon, aluminum, aluminum silicate, aluminum oxide, titanium, magnesium, steel, rockwool, and tungsten. Illustrative of polymeric fibers which can be used in known composites are those composed of rayon, nylon, kevlar, polyesters and polyolefins.
Several methods are described in the prior art for forming polymer fibers. Illustrative of such methods are melt spinning, wet spinning, and dry spinning. Other less common processes are reaction spinning, dispersion spinning, and emulsion spinning. Representative of publications which describe these methods are: H.F. Mark, S.M. Atlas, and E. Cernia, "Man-Made Fibers," Volume 1-3 (1967) and J.E. McIntyre, "Man-Made Fibers, Manufacturer," in Encyclopedia of Polymer Science and Technology, Volume 8, p. 374, by H.F. Mark and N.M. Bikales. One important common feature in these prior art spinning processes is the use of a spinneret which is a plate containing orifices through which the molten or dissolved polymer is extruded and shaped under pressure. Several techniques are described in the prior art for fabrication of polymer short fibers. For example, U.S. Pat. No. 2,988,782 describes such a procedure in which such fibers are formed by precipitation and violent agitation. U.S. Pat. Nos. 4,125,584 and 4,178,336 describe another procedure in which such short fibers are formed by centrifugal spinning. All the short fiber manufacturing processes described above have one important feature in common. In each of these processes, the fiber-forming material must be converted to liquid or solution, either by melting or dissolution, so that it can be transferred by a pump or extruder to a spinneret for fiber spinning.
Polymers capable of forming anisotropic melts are known. By "a polymer capable of forming an anisotropic melt" is meant that the polymer forms such a melt when heated to a particular temperature range, characteristic of the polymer (this type is termed a "thermotropic" polymer), or can be induced to form such a melt by the application of shear to the melt. The latter state is characterized by the persistence of the anisotropic condition for a period of a second or two after the melt ceases to be sheared. This distinguishes it from the well-known observation that a polyethylene terephthalate melt will exhibit order when sheared by passing the melt through a tube. Such order disappears immediately once the melt ceases to be sheared. Some polymers may show both thermotropic and shear-induced anisotropy.
Illustrative of polymers capable of forming anisotropic melts are wholly aromatic polyesters as for example the aromatic polyesters described in Belgian Pat. Nos. 828,935 and 828,936; Dutch Pat. No. 7505551; West German Pat. Nos. 2,520,819, 2,520,820 and 2,722,120; Japanese Pat. Nos. 43-223, 2132-116, 3017-692 and 3021-293; U.S. Pat. Nos. 4,118,372, 3,991,013, 3,991,014, 4,057,597, 4,284,757, 4,107,143, 4,066,620, 4,075,262, 3,991,013, 4,140,846, 4,156,070, 4,159,365; 4,169,933, 4,181,792, 4,067,852, 4,083,829, 4,130,545, 4,161,470, 4,284,757, 4,184,996, 4,238,559, 4,238,598, 4,230,817, and 4,256,629; and UK Application No. 2,002,404.
Still other polyesters which exhibit melt anisotropy are those described in Polyester X7G-A Self Reinforced Thermoplastic, by W.J. Jackson, Jr., H.F. Kuhfuss, and T.F. Gray, Jr. 30th Anniversary Technical Conference, 1975 Reinforced Plastics Composites Institute, The Society of the Plastics Industry, Inc., Section 17-D, Pages 1-4; Belgian Pat. Nos. 838,935 and 828,936; Dutch Pat. No. 7505551; West German Pat. Nos. 2520819, 2520820, 2722120, 2834535, 2834536 and 2834537; Japanese Pat. Nos. 43-223, 2132-116, 3017-692 and 3021 293; U.S. Pat. Nos. 4,093,595, 4,130,702, 4,146,702, 4,153,779, 4,181,792, 4,183,895, 4,188,476, 4,201,856, 4,219,461, 4,224,433, 4,226,970, 4,232,143, 4,232,144, 4,238,598, 4,238,600, 4,242,496, 4,245,082, 4,245,084, 4,247,514, 4,265,802, 4,267,304, 4,269,965, 4,272,625, 4,279,803, 4,285,852, 4,287,332, 4,294,955, 4,299,756, 4,314,073, 4,318,841, and 4,318,842, and U.K. application No. 2,002,404.
Representative disclosures of anisotropic melt forming polyesters or poly(ester-amides) which may include 2,6-dicarboxynaphthalene moiety and/or 2,6-dioxynaphthalene moiety are present in U.S. Pat. Nos. 4,067,852; 4,083,829; 4,093,595; 4,118,372; 4,130,545; 4,156,070, 4,169,933; 4,181,792; 4,184,966; 4,188,476; 4,201,856; 4,242,496; 4,247,514; 4,294,955; 4,339,375, and 4,355,134.
Illustrative of polyazomethines which are capable of forming anisotropic melts are disclosed in U.S. Pat. No. 4,048,148.
Blends of melt processable polymers and polymers which form anisotropic melts are known. For example, U.S. Pat. No. 4,565,850 describes a blend of a thermoplastic polymer and an oligomer which exhibits thermotropic behavior at certain temperatures. Similarly, U.S. Pat. No. 4,386,174 describes a composition containing a melt processable polymer and a sufficient quantity of a polymer capable of forming an anisotropic melt at a temperature within the processing temperature range of the melt processable polymer. U.S. Pat. No. 4,611,025 discloses a process for forming a blend of a thermoplastic polymer and a thermotropic oligomer.
U.K. Published Patent Application 2,008,598 discloses a polymer composition comprising 20 percent or less, based on the total weight of polymeric material, of a first rigid polymeric material with the balance being a second polymeric material composed substantially of flexible molecular chains. The first polymeric material is dispersed in the second polymeric material in a microscopic region of 1 .mu.m. or less. Foreign counterparts of this application include Japanese 54065747, French 2407956 and West German (BRD) 2847783.
U.S. Pat. No. 4,460,735 and its foreign counterpart EPO Patent Application No. 0 044 175 describes polymer blends comprising approximately 75 percent by weight, based upon the total weight of the blend, of a polycarbonate and approximately 25 to approximately 95 percent by weight, based upon the total weight of the blend of a melt processable wholly aromatic polyester which is capable of forming an anisotropic melt phase apart from the blend.
Short fibers are frequently used as a reinforcing component of various types of composites. Representative of these reinforcing fibers include asbestos fibers, glass fibers, boron fibers, graphitic carbon fibers, amorphous carbon fibers, synthetic polymeric fibers, aluminum fibers, aluminum silicate fibers, oxides of aluminum fibers, titanium fibers, magnesium fibers, rockwool fibers, steel fibers, tungsten fibers, cotton fibers, wool fibers, wood cellulose fibers and the like.